This invention relates generally to modulated cavity oscillators and more specifically to means for minimizing modulation nonlinearities in a broad band cavity oscillator.
There are various types of known frequency modulated microwave cavity oscillators. A Gunn diode mounted within a cavity oscillator has been utilized to provide the necessary amplification to sustain oscillation. A varactor diode also mounted within the cavity can be used to vary the oscillator frequency under the control of an external voltage source thereby frequency modulating the oscillator.
Various types of microwave cavity oscillators are known wherein two coupled cavities are employed. In U.S. Pat. No. 3,803,513 two cavity resonators having different resonant frequencies are coupled together by means of hybrid circuit to provide stable operation relative to temperature variations. In order to achieve a low noise high power output as taught in U.S. Pat. No. 3,858,123, two wave guide sections are coupled together by a directional resonator.
Secondary cavities associated with a main cavity have also been utilized to provide certain desired characteristics. For example, in U.S. Pat. No. 3,913,035 a high-Q secondary cavity is connected to a primary low-Q cavity in order to enhance frequency stablization and eliminate noise associated with spurious signals. U.S. Pat. No. 3,883,822 discloses a load terminated auxillary cavity which is coupled to a main cavity which controls the wave mode and frequency in order to minimize the risk of parasitic oscillations. A secondary cavity terminated in an adjustable short has been used with a primary cavity to effect the tuning of harmonics; see Ezio M. Bastida "Study of Harmonic Effects for Waveguide Gunn--Diode Oscillator Optimization," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-28, No. 4 (April 1980), 305-313.
Modulation nonlinearities due to undesired higher order resonances in frequency modulated microwave cavity oscillators have presented a difficult problem. One technique used in attempting to solve this problem is to insert lossy ferrite materials inside the resonant cavity to de-Q undesired modes. However, such ferrites also tend to de-Q the fundamental, that is the desired, resonant frequency. Another technique that has been utilized to combat the modulation nonlinearity problem is to employ special tuning elements in the resonant cavity such that they effect the harmonic frequency more than the fundamental. Such tuning elements are adjusted to minimize the nonlinearity attributed to harmonics at a certain fundamental frequency. However, this technique has the objectional feature of requiring numerous adjustments for a given operating frequency and may require readjustments should the operating frequency be changed.
U.S. Pat. No. 3,982,211 to G. G. Endersz discloses a frequency modulated cavity oscillator having improved linearity. The improved linearity is achieved by a parallel resonant circuit tuned to a second harmonic of the fundamental frequency. This technique has the disadvantage of having to retune the circuit resonant at the second harmonic whenever the fundamental operating frequency is changed. This technique is also addressed in an article "Linearity Improvement of Microwave FM Oscillators by Haromonic Tuning" by G. G. Endersz in IEEE Transactions on Microwave Theory and Techniques Vol. MTT-23, No. 4 (April 1975), 360-367.